Method and apparatus for split download

ABSTRACT

A download method forms a download user network with a plurality of user terminals which are going to download the same data. The delivery server divides the data into data portions, and assigns the data portions to the respective user terminals belonging to the download user network. Then each of the user terminals downloads the assigned data portion from the delivery server. Thereafter the downloaded data portions are mutually transmitted among the user terminals within the download user network so that each of the user terminals can obtain all the data portions which constitute the data. Each of the user terminals combines the obtained data portions into the original data. Thus the data is efficiently downloaded to the plurality of user terminals.

CROSS REFERENCE TO RELATED APPLICATION

The present application is based on and incorporates herein by referenceJapanese Patent Application No. 2000-330889 filed on Oct. 30, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a download method, and a user terminaland a delivery server therefor.

2. Related Art

JP-A-H7-30573 proposes a download method for downloading the same datato all slave stations (user terminals). The method first sends adownload request including a data identifier from a master station(sender) to slave stations which are directly connected to the masterstation. Each of the slave stations which have received the downloadrequest determines whether to accept or reject the download requestbased on the data identifier. The master station sends the datacorresponding to the data identifier to the slave stations which haveaccepted the download request. Each of the slave stations which havereceived the data sends the received data in turn to slave stationswhich are connected thereto. This process is repeated, so that the samedata is downloaded to all the slave stations.

This method downloads the data from the master station to the slavestations which are directly connected thereto via a wide area network(WAN), and sends the data to the rest of the slave stations via a localarea network (LAN) in parallel. Therefore the total time required fordownloading the same data to all the slave stations is reduced incomparison with a conventional method which downloads the data from amaster station directly to all slave stations.

However, according to JP-A-H7-30573, the master station sends the entiredata to each of the slave stations directly connected thereto. Thereforea relatively large amount of data is sent via the WAN. That is, it stilltakes a relatively long time to download the same data to all the slavestations.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a download methodwhich efficiently downloads the same data to a plurality of userterminals.

It is also an object of the present invention to provide a user terminaland a delivery server for implementing the download method.

A download method according to the present invention forms a downloaduser network with a plurality of user terminals which are going todownload the same data. A delivery server divides the data into dataportions, and assigns the data portions to the respective user terminalsbelonging to the download user network. Then each of the user terminalsdownloads the assigned data portion from the delivery server. Thereafterthe downloaded data portions are mutually transmitted among the userterminals within the download user network so that each of the userterminals can obtain all the data portions which constitute the data.

Alternatively, one of the user terminals belonging to the download usernetwork may download the entire data from the delivery server andtransmit the downloaded data to each of the other user terminals.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription made with reference to the accompanying drawings. In thedrawings

FIG. 1 is a block diagram showing a download system for implementing adownload method according to an embodiment of the present invention;

FIG. 2 is a pictorial diagram showing the download system of FIG. 1;

FIGS. 3A–3F are schematic diagrams showing the procedure of the downloadmethod implemented by the download system of FIG. 1;

FIG. 4 is a flowchart of the download method implemented by the downloadsystem of FIG. 1;

FIG. 5A is a schematic diagram showing a data split process executed bya delivery server;

FIG. 5B is a diagram showing the content of a split download mapattached to each group of data fragments in the data split process;

FIG. 6 is a schematic diagram showing the procedure of one-to-one mutualtransmission executed among user terminals;

FIG. 7 is a block diagram showing the construction of the deliveryserver;

FIG. 8 is a block diagram showing the construction of each of the userterminals;

FIG. 9 is a schematic diagram showing the procedure of one-to-multiplemutual transmission executed among the user terminals;

FIG. 10 is a block diagram showing a media data download system;

FIG. 11 is a pictorial diagram showing the media data download system ofFIG. 10; and

FIGS. 12A and 12B are flowcharts of the first half and the second halfof the media data download method implemented by the media data downloadsystem of FIG. 10, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 1 and 2, a download system for implementing adownload method according to the present invention includes a deliveryserver 1 owned by a service provider, base stations 41, 42, and userterminals (UT) 21–29 such as mobile phones. The user terminals 21–29 canperform radio telecommunication with the delivery server 1 via the basestations 41, 42 and a wide area network (WAN) 5.

Further each of the user terminals 21–29 can form a download usernetwork 31–33, which is other than the WAN 5, with other user terminals21–29 within a cell for local radio communication. The download usernetwork 31 is an ad hoc decentralized autonomous network formed with aplurality of user terminals among which high-speed local radiocommunication can be performed via a wireless LAN.

The download system downloads the same data (e.g., broadcastinformation) from the delivery server 1 to some of the user terminals21–29 as follows. Users each purchase a terminal and make a contractwith the service provider for communications service beforehand. The ID(the unique number or address) of the user terminal 21–29 is registeredwith the delivery server 1 at the time of the contract. The serviceprovider informs the users of the contents and time of the service to beprovided and the charge for the service from the delivery server 1 viathe WAN 5 or by other information providing means. The users each selectinformation to be downloaded, and register with the delivery server 1 asa subscriber to the selected information.

Referring to FIG. 4, at step 701, the delivery server 1 transmits astart signal (download request) to the user terminals (subscriberterminals) 21–29 registered as subscribers to data via the WAN 5 andbase stations 41, 42 at a time to start delivering the data as shown inFIG. 3A.

Each of the subscriber terminals 21–29 receives the download request atstep 702, and notifies other user terminals 21–29 that it is going todownload the data. This notification is transmitted using carrier sensemultiple access technique via the wireless LAN within the cell for localradio communication. Specifically, the subscriber terminals 21–29 eachtransmit, at random timing, a signal (participation signal) includingits terminal ID and information that it is going to download the data.The user terminals 21–29 in the local communication cell receive theparticipation signals from the subscriber terminals 21–29 at randomtiming. Thus the subscriber terminals 21–29 each search for othersubscriber terminals 21–29 within the local communication cell.

When a predetermined time elapses from the beginning of the transmissionof the participation signals, the participation list is closed. Then thesubscriber terminals 21–29 each obtain information including the numberof the subscriber terminals 21–29 which are going to download the samedata and the IDs of such subscriber terminals 21–29. At step 703, basedon the obtained information, the subscriber terminals 21–29, which aregoing to download the same data and included in the same localcommunication cell, form a download user network 31–33 as shown in FIG.3B.

Thus, a plurality of download user networks 31–33 are formed as shown inFIG. 1 or 2. However FIGS. 3A–3F show only one download user network 31as an example, and therefore the operation of the present system will behereinafter explained only about the example. The user terminals 24–29belonging to the other download user networks 32, 33 operate similarlyto the user terminals 21–23 belonging to the download user network 31,and the delivery server 1 also executes the similar processes for theother download user networks 32, 33.

Each of the user terminals 21–23 includes software for managing itsoperation in the download user network 31, because the download usernetwork 31 does not include a base station which controls thecommunication among the user terminals 21–23 in the download usernetwork 31.

One of the user terminals 21–23 belonging to the download user network31 is arbitrarily selected as a representative user terminal (virtualadministrator). For example, the user terminal 21 is selected as therepresentative user terminal (RUT) as shown in FIG. 3C. At step 704, therepresentative user terminal 21 transmits the IDs of the user terminals21–23 belonging to the download user network 31 with a downloadacceptance signal to the delivery server 1 via the base station 41 andthe WAN 5 as shown in FIG. 3C.

At step 705, the delivery server 1 obtains the number of the userterminals 21–23 belonging to the download user networks 31 from the userterminal IDs received from the representative user terminals 21. Thenthe delivery server 1 divides the data to be delivered among the userterminals 21–23 belonging to the download user networks 31.

Specifically, referring to FIG. 5A, the delivery server 1 divides thedata into a predetermined number of fragments beforehand, and attaches aheader to each of the fragments. The header includes data identificationinformation, information (serial number) on the position of the fragmentin the entire data, and error detecting code (CRC or the like). Thepredetermined number is set to a positive integer (e.g., 60) which isdivisible by relatively many integers. When the number of the userterminals belonging to the download user network is ‘4’ for example,fifteen data fragments are transmitted to each of the user terminals.

In FIG. 5A, assuming that the data is divided among three user terminals21–23 of the download user network 31, the data fragments are dividedinto three groups. Then a split download map is attached to each of thegroups. The split download map includes data identification information,the identifier of the download user network, a database including dataon the number and position of the data fragments assigned to therespective user terminal (that is, the serial numbers of the datafragments assigned to the respective user terminal), the number ofupdates of the split download map, and error detecting code (CRC or thelike) as shown in FIG. 5B.

The groups of the data fragments are transmitted to the respective userterminals 21–23 via the WAN 5 as shown in FIG. 3D. The user terminals21–23 each receive the assigned data fragments at step 706, and verifythe received data fragments based on the error detecting codes. When anerror is detected, the user terminal 21–23 requests the delivery server1 to retransmit the assigned data fragments.

If another user terminal newly joins the download user network 31 afterthe download acceptance signal is transmitted, the representative userterminal 21 notifies the delivery server 1 of the new user terminal.Then the delivery server 1 dynamically changes the number of the datafragments assigned to each of the user terminals so that the datafragments are also assigned to the new user terminal. Then the newlyassigned data fragments are transmitted to each of the user terminals21–23.

If one of the user terminals belonging to the download user network 31is disconnected from the delivery server 1, the delivery server 1detects the disconnection or the representative user terminal 21notifies the delivery server 1 of the disconnection. Then the deliveryserver 1 dynamically changes the number of the data fragments assignedto each of the user terminal so that no data fragment is assigned to thedisconnected user terminal. Then the newly assigned data fragments aretransmitted to each of the user terminals 21–23.

The representative user terminal 21 checks whether all the userterminals 21–23 belonging to the download user network 31 have alreadydownloaded the assigned data fragments. When the representative userterminal 21 determines that all the user terminals 21–23 have alreadydownloaded the assigned data fragments, it plans mutual transmissionamong the user terminals 21–23, and instructs the user terminals 22,23to start one-to-one mutual transmission within the download user network31 as shown in FIG. 3E at step 707.

Referring to FIG. 6, assuming that the six user terminals A–F havedownloaded the respective group a–f of the data fragments, theone-to-one mutual transmission is performed as follows. At the firststage, the user terminals A, B exchange the data fragments ‘a’, ‘b’, theuser terminals C, D exchange the data fragments ‘c’, ‘d’, and the userterminals E, F exchange the data fragments ‘e’, ‘f’. The split downloadmaps of the user terminals A–F are updated so that the data exchange atthe first stage is reflected.

At the second stage, based on the updated split download maps, the userterminals A, C exchange the data fragments ‘ab’, ‘cd’, the userterminals B, E exchange the data fragments ‘ab’, ‘ef’, and the userterminals D, F exchange the data fragments ‘cd’, ‘ef’. The splitdownload maps of the user terminals A–F are updated so that the dataexchange at the second stage is reflected.

At the final stage, based on the updated split download maps, the userterminals A, F exchange the data fragments ‘ab’, ‘ef’ except theoverlapping data fragments ‘cd’, the user terminals B, C exchange thedata fragments ‘ef’, ‘cd’ except the overlapping data fragments ‘ab’,and the user terminals D, E exchange the data fragments ‘cd’, ‘ab’except the overlapping data fragments ‘ef’. Thus all the user terminalsA–F obtain the entire data ‘abcdef’.

The user terminals, which are at a relatively long distance from oneanother within the download user network, sometimes cannot communicatewith one another via the wireless LAN. In the case that the mutualtransmission should be performed between such user terminals, the userterminals each obtain the required data fragments from a nearer userterminal, which has the required data fragments, instead. Further, theuser terminal, which has been disconnected from the delivery server 1,can obtain the data from the other user terminals during the mutualtransmission.

The download user network is omnidirectionally established, andtherefore any user terminal can omnidirectionally communicate with allthe other user terminals within the download user network. Accordingly,the data exchange between one pair of the user terminals should beperformed at a time, and thereafter the data exchange between anotherpair of the user terminals should be performed after a predeterminedinterval at each stage of the mutual transmission. Thus the dataexchange between each pair of the user terminals is performed withoutinterference with signals from other user terminals. In this case, themutual transmission shown in FIG. 6 is accomplished by performing thecommunication between a pair of the user terminals A–F nine (3 pairs×3stages) times.

If the download user network includes a mechanism for directionalcontrol of communication among the user terminals, any user terminal candirectionally communicate with one of the other user terminals at a timealthough the download user network is omnidirectionally established. Inthis case, because the communication can be simultaneously performedbetween disjoint pairs of the user terminals, the mutual transmissionshown in FIG. 6 is accomplished by performing the simultaneouscommunication between three pairs of user terminals A–F only threetimes. However, the complicated mechanism for directional control of thecommunication is required, and further time for executing processes forthe directional control is also required.

The split download maps of a pair of the user terminals A–F are updatedimmediately after the data exchange between the pair is completed.Therefore the contents of the split download maps of the user terminalsA–F are not consistent with one another in the middle of each stage ofthe mutual transmission. In order to keep the split download mapsconsistent at all times, communication specially for updating the splitdownload maps should be performed whenever the data exchange between apair of the user terminals A–F is completed.

When each of the user terminals 21–23 belonging to the download usernetwork 31 obtains all the data fragments by the mutual transmission, itcombines the obtained data fragments referring to the headers attachedthereto at step 708. When the data fragments are combined into theoriginal data, the user terminal 21–23 transmits a notification ofcompletion of the mutual transmission to the representative userterminal.

When the representative user terminal 21 receives the notifications ofcompletion of the mutual transmission from all the user terminals 21–23belonging to the download user network 31, it transmits a notificationof completion of the download to the delivery server 1 as shown in FIG.3F and dissolves the download user network 31 at step 709. When thedelivery server 1 receives the notification of completion of thedownload from all the representative user terminals, that is, from allthe download user networks 31–33, the download of the data from thedelivery server 1 to the plurality of the user terminals 21–29 iscompleted.

The construction of the delivery server 1 is as follows. Referring toFIG. 7, the delivery server 1 is formed with a computer and includesstorage means 11 for storing various data such as data to be delivered,communication means 12 for communicating with the user terminals 21–29via the WAN 5, and processing means 13 for executing a program which isstored beforehand and implements various processes required fordelivering the data.

The processing means 13 includes, for example, means for dividing thedata to be delivered into data fragments and attaching a header to eachof the data fragments, means for instructing the communication means 12to transmit the download request, means for assigning the data fragmentsto the user terminals according to the number of the user terminalsbelonging to the download user network, means for instructing thecommunication means 12 to transmit the assigned data fragments to theuser terminals, means for dynamically changing the number of the datafragments assigned to each of the user terminals in response to changeof the number of the user terminals belonging to the download usernetwork or existence of a user terminal disconnected from the deliveryserver 1, and means for removing the delivered data from the storagemeans 11 in response to the notification of completion of the download.

The construction of each of the user terminals 21–29 is as follows.Referring to FIG. 8, each of the user terminals 21–29 is formed with acomputer and includes first communication means 201 for communicatingwith the delivery server 1 via the WAN 5, second communication means 202for communicating with other user terminals 21–29 at a high transmissionrate via the wireless LAN, storage means 203 for storing downloadeddata, and processing means 204 for executing a program which is storedbeforehand and implements various processes required for downloadingdata.

The processing means 204 includes, for example, means for controllingthe operation of the user terminal in the download network, means forinstructing the second communication means 202 to transmit theparticipation signal for participating in the download user network,means for exchanging various information with the delivery server 1 whenthe user terminal is selected as the representative user terminal, meansfor performing the mutual transmission with other user terminals, andmeans for combining the obtained data fragments into the original data.

In the present embodiment, the total amount of the data transmitted toeach of the local communication cells via the WAN 5 is reduced to thesame amount as the entire data. The mutual transmission via the LANcompensates for the reduction in the amount of the data transmitted viathe WAN 5. Therefore the data is efficiently downloaded to the pluralityof user terminals.

(Modifications)

In the above embodiment, at step 702, each of the user terminals 21–23which have received the download request from the delivery server 1 mayattach information (e.g., received power) on how well the user terminal21–23 can communicate with the delivery server 1 via the WAN 5 to theparticipation signal. Then the user terminal which can communicate withthe delivery server 1 best may be selected as the representative userterminal.

Further in this case, the representative user terminal 21 may attach theinformation on how well each of the user terminals 21–23 can communicatewith the delivery server 1 via the WAN 5 to the download acceptancesignal at step 704. Then the delivery server 1 may assign the differentnumber of the data fragments to each of the user terminals based on theinformation at step 705 so that the download of the data is preventedfrom being prolonged. Moreover, at step 705, the delivery server 1 mayassign the data fragments to the respective user terminals 21–23 basedon the information so that the assigned data fragments overlap eachother.

Further in the above embodiment, the download user network 31 may beformed beforehand with the user terminals 21–23 which are going todownload the same data. That is, step 701 may be omitted and steps 702,703 may be modified and executed beforehand as follows. Each of the userterminals 21–23 transmits the participation signal within the localcommunication cell, when it registers with the delivery server 1 as asubscriber to the data. The participation list is closed at some pointof time and the download user network 31 is formed with the userterminals 21–23 in the participation list. Further the representativeuser terminal 21 is selected. Thereafter a new user terminal may jointhe download user network 31 by transmitting the participation signal.

In this case, step 704 is performed at a time to start downloading thedata as follows. The representative user terminal 21 transmits a startsignal (download request) with the IDs of all the user terminals 21–23belonging to the download user network 31 to the delivery server 1 viathe WAN 5. That is, the download request is transmitted from therepresentative user terminal 21 to the delivery server 1 in this case,while it is transmitted from the delivery server 1 to the user terminals21–23 in the above embodiment.

Further in the above embodiment, at step 705, the delivery server 1 maydirectly divide the entire data according to the number of the userterminals 21–23 belonging to the download network 31 without dividingthe entire data into the predetermined number of the data fragmentsbeforehand. In this case, although the data should be dividedimmediately before the data fragments are transmitted to the userterminals 21–23, the data can be evenly assigned to each of the userterminals 21–23 belonging to the download user network 31.

In the above embodiment, steps 705–707 may be modified as follows. Thedelivery server 1 transmits the entire data to the representative userterminal 21 without dividing the data. Then the representative userterminal 21 receives the entire data via the WAN 5, and transmits thereceived entire data to each of the user terminals 21–23 belonging tothe download network 31. Step 708 is bypassed in this case. In thepresent modification, the total amount of the data transmitted to eachof the local communication cells via the WAN 5 is also reduced to thesame amount as the entire data. Therefore the data is efficientlydownloaded to the plurality of user terminals.

During the download of the entire data from delivery server 1, therepresentative user terminal 21 may sequentially extract a data fragmentfrom an already downloaded portion of the data and transmit the datafragment to each of the user terminals 21–23 immediately after theportion of the data is received. In this case, the time required fordownloading the entire data to the plurality of user terminals isfurther reduced.

Further, steps 705–707 of the above embodiment may be modified asfollows. The delivery server 1 divides the entire data into datafragments at step 705, and sequentially transmits the data fragments tothe representative user terminal 21. During the download of the datafrom the delivery server 1, the representative user terminal 21transmits every data fragment to each of the other user terminals 22, 23immediately after the data fragment is received.

If the representative user terminal 21 cannot communicate with thedelivery server 1 well via the WAN 5 in the above modification, thedelivery server 1 may transmit all the data fragments which constitutethe entire data to one of the other user terminals 22,23 which cancommunicate with the delivery server 1 better than the representativeuser terminal 21. Then, the user terminal which has received all thedata fragments transmits the received data fragments to all the otheruser terminals including the representative user terminal 21.

In the above embodiment, each of the user terminals 21–23 may severallystart the mutual transmission immediately after the user terminal 21–23completes the download of the assigned data fragments from the deliveryserver 1, even if all the user terminals 21–23 do not complete thedownload of the assigned data fragments yet. That is, step 707 may bestarted before step 706 is completed. Then, the total time required fordownloading the entire data to the plurality of the user terminals isfurther reduced.

In the case that the one-to-one mutual transmission is performed as theabove embodiment, the amount of the data exchanged between a pair of theuser terminals at a time is often more than that of the data downloadedto each of the user terminals from the delivery server 1 especially ateach of the stages after the first stage, even if the data fragmentsexcept the overlapping data fragments are only exchanged. Further, thecontents of the split download maps of the user terminals are notconsistent with one another in the middle of each stage of the mutualtransmission as described above. For overcoming these drawbacks,one-to-multiple mutual transmission may be performed in the aboveembodiment instead of the one-to-one mutual transmission as follows.

Referring to FIG. 9, assuming that the six user terminals A–F havedownloaded the respective group a–f of the data fragments, the datafragments ‘a’ are transmitted from the user terminal A to the other userterminals B–F at the first stage. The data fragments ‘b’ are transmittedfrom the user terminal B to the other user terminals A, C–F at thesecond stage. The data fragments ‘c’ are transmitted from the userterminal C to the other user terminals A, B, D–F at the third stage. Thedata fragments ‘d’ are transmitted from the user terminal D to the otheruser terminals A–C, E, F at the fourth stage. The data fragments ‘e’ aretransmitted from the user terminal E to the other user terminals A–D, Fat the fifth stage. The data fragments ‘f’ are transmitted from the userterminal F to the other user terminals A–E at the final stage.

Any user terminal must be able to communicate omnidirectionally with allthe other user terminals within the download user network in order toperform the one-to-multiple mutual transmission. In the case of theone-to-multiple mutual transmission, high transmission efficiency isachieved because the data fragments are transmitted to a plurality ofuser terminals at a time. Further the contents of the split downloadmaps of the user terminals A–F are kept consistent with one another atall times. Moreover only the same number of the data fragments as thatdownloaded to each of the user terminals A–F is transmitted at everystage. However, the communication should be performed the same number oftimes as that of the user terminals A–F.

In the above embodiment, the representative user terminal 21 does nothave to notice the delivery server 1 of the new user terminal when thenew user terminal joins the download user network 31 after the downloadacceptance signal is transmitted. That is, the new user terminal mayobtain all the data fragments which constitute the entire data from theother user terminals 21–23 when the mutual transmission is performedamong the user terminals 21–23.

Application of Embodiments to Delivery of Media Data

Various information can be downloaded to a plurality of user terminalsaccording to the above embodiment or modifications. For example, mediadata such as music is downloaded as follows. Referring to FIGS. 10 and11, a media providing company includes media databases 61, 62, 63 whichstore the media data. A media delivery company includes the deliveryserver 1 and communication equipment. The media providing company andthe media delivery company need not to be separate, that is, they may bethe same company.

Referring to FIG. 12, the media delivery company provides terminals andcommunications service. Users each purchase a terminal and make acontract with the media delivery company to subscribe to thecommunications service at step 801. The basic charge to be paid by theuser is fixed at the time of the contract. The media delivery companycooperates with the media providing company at step 802. Then the mediaproviding company provides the data to be delivered for the mediadelivery company. The media delivery company pays the media providingcompany for the data. The media delivery company provides information onmedia data to be delivered for the users. The users each select the datato be downloaded based on the information, and make a contract with themedia delivery company to subscribe to the selected data at step 803.

Referring to FIG. 12B, the media delivery company provides informationon the subscription for the media providing company as information onconsumers' behavior at step 804, and the media providing company maygive a discount off the price of the data in return. Then the discountgiven by the media providing company leads to reduction of the mediarate to be paid by the user. The information on the consumers' behavioris reflected in marketing strategy in the media providing company.

The delivery server 1 receives data to be delivered from the mediadatabases 61, 62, 63. Thereafter the delivery server 1 delivers the dataaccording to the above embodiment or modifications at step 805. Theaccounting information on the delivered data is accumulated in the mediadelivery company when the data is delivered. The media delivery companybills the users for the delivered data and the communication, and theusers each pay the bill at step 806.

In the case that the data is downloaded according to the aboveembodiment, the total charge F may be calculated based on the amount ofthe downloaded data using the following formula, for example:F=αf _(D1) D+βf _(D2) D/nwhere ‘n’ is the number of user terminals among which the entire data isdivided, ‘D’ is the amount of the entire data, ‘f_(D1)’ is the charge(copyright royalty) for unit data, ‘f_(D2)’ is the charge forcommunication of the unit data, ‘α’ is the discount rate for the data,and ‘β’ is the discount rate for the communication. The first term‘αf_(D1)D’ of the formula corresponds to the charge for the data, andthe second term ‘βf_(D2)D/n’ corresponds to the charge for thecommunication.

In the present media data download system, the media providing companyhas the merits of being capable of increasing the customers utilizingthe sales network of the media delivery company and obtaining theinformation on the customers' behavior as described above. These meritsare reflected in the charge for the data as the discount rate α.

On the other hand, the media delivery company has the merit of beingcapable of increasing the number of the subscribers to the communicationdue to the increase in the number of the subscribers to the media data.Furthermore the media delivery company has the merit of efficient use ofthe communication equipment due to reduction in transmission time of thedata. If the delivery of the data is performed during low-traffic hours,the efficiency in the use of the communication equipment is furtherimproved. These merits are reflected in the charge for the communicationas the discount rate β.

Moreover, assuming that the communication within the download usernetwork is free of charge, the user has the merit of being charged lessfor the communication, because the charge for the communication‘βf_(D2)D’ is divided by the number ‘n’ of the user terminals in theabove formula.

Alternatively, the total charge F may be calculated based on the amountof the downloaded data and the time taken for downloading the data usingthe following formula, for example:F=αf _(D1) D+βf _(t2) t/nwhere ‘n’ is the number of user terminals among which the entire data isdivided, ‘t’ is the time taken for downloading the data, ‘f_(D1)’ is thecharge (copyright royalty) for unit data, ‘f_(t2)’ is the charge forcommunication of the unit time, ‘α’ is the discount rate for the data,and ‘β’ is the discount rate for the communication. The first term‘αf_(D1)D’ of the formula corresponds to the charge for the data, andthe second term ‘βf_(t2)t/n’ corresponds to the charge for thecommunication.

When this charging system is employed, the time required for downloadingthe data from the delivery server 1, that is, the transmission rateshould be the same in all the user terminals. If the transmission rateis not the same, the total charge for the same data varies from one userterminal to another.

A charging system other than the above charging systems may be employed.For example, the charge for the data is fixed, and the total charge maybe calculated by adding the charge depending on the amount of the dataand the charge depending on the time for downloading the data to thefixed charge for the data.

1. A download method for downloading data from a delivery server to aplurality of user terminals, comprising the steps of: forming a downloaduser network with the plurality of user terminals to download said datato the plurality of user terminals; said delivery server dividing saiddata into a plurality of data portions, each of said plurality of dataportions being assigned to a respective user terminal; downloading eachof said assigned data portions from said delivery server to each of saidrespective user terminals; and transmitting each of said plurality ofdata portions among said plurality of user terminals so that each ofsaid plurality of user terminals receive said data by receiving each ofsaid plurality of data portions.
 2. A download method as in claim 1,wherein said dividing step is performed according to a number of saidplurality of user terminals.
 3. A download method as in claim 2, saidforming step further comprising the steps of: selecting one of saidplurality of user terminals as a representative user terminal; and saidrepresentative user terminal transmitting information from which thenumber of said plurality of user terminals is obtained to said deliveryserver.
 4. A download method as in claim 3, said forming step furthercomprising the steps of: said representative user terminal gatheringcommunication environment information on each of said plurality of saiduser terminals; and said representative user terminal transmitting saidgathered communication environment information to said delivery server,wherein said dividing step is performed according to said communicationenvironment information.
 5. A download method as in claim 3, whereinsaid representative user terminal notifies said delivery server of anaddition of a new user terminal to said download user network if saidnew user terminal joins said download network between completion of saidforming step and beginning of said mutual transmitting step, whereinsaid delivery server redivides said data into a plurality of new dataportions in response to the addition of said new user terminal so thatsaid plurality of new data portions are reassigned to said new userterminal and said plurality of said user terminals, respectively, andwherein said downloading step is achieved by downloading said reassigneddata portions from said delivery server to said new user terminal andsaid plurality of user terminals, respectively.
 6. A download method asin claim 3, wherein a new user terminal obtains said plurality of dataportions which constitute said data from said plurality of userterminals during said mutual transmitting step if said new user terminaljoins said download user network between completion of said forming stepand beginning of said mutual transmitting step.
 7. A download method asin claim 3, wherein if one of said plurality of user terminals isdisconnected from said delivery server between completion of saidforming step and beginning of said mutual transmitting step, saiddelivery server redivides said data into a plurality of new dataportions so that said plurality of new data portions are reassigned tosaid plurality of user terminals except said disconnected user terminal,and wherein said downloading step is achieved by downloading saidplurality of reassigned data portions from said delivery server to saidplurality of user terminals except said disconnected user terminal,respectively.
 8. A download method as in claim 3, wherein saidrepresentative user terminals gives said plurality of user terminals aninstruction to start the mutual transmitting step when saidrepresentative user terminal determines that said downloading step iscompleted in all of said plurality of user terminals, and wherein saidmutual transmitting step is started simultaneously in all of saidplurality of user terminals in response to said instruction.
 9. Adownload method as in claim 4, wherein said dividing step is performedso that said data portions overlap each other according to saidcommunication environment information.
 10. A download method as in claim1, wherein said mutual transmitting step is started severally in each ofsaid plurality of user terminals when said downloading step is completedin the user terminal.
 11. A download method as in claim 1, said dividingstep further comprising the steps of: generating a split download mapincluding information on correspondence between said plurality of userterminals and said data portions assigned thereto; and attaching saidsplit download map to each of said data portions, wherein said splitdownload map is downloaded with each of said data portions to each ofsaid plurality of user terminals in said downloading step.
 12. A userterminal for downloading data from a delivery server comprising: meansfor forming a download user network with at least one other userterminal to download said data to said user terminal and to said otheruser terminal; means for downloading a first data portion of said datafrom a delivery server to said user terminal and a second data portionof said data to said other user terminal; means for transmitting saidfirst data portion to said other user terminal and receiving said seconddata portion of said data from said other user terminal; and means forcombining said first and second data portions into said data at saiduser terminal and at said other user terminal.
 13. A download system fordownloading same data to a plurality of user terminals comprising: aplurality of user terminals capable of forming a local area network; anda delivery server capable of communicating with said plurality of userterminals, wherein said data is divided into a plurality of datafragments and each of said plurality of data fragments are assigned to arespective user terminal; and wherein each of said plurality of userterminals downloads its respective assigned data fragment from saiddelivery server, and exchange said downloaded data fragments with oneanother via said local area network so that each of said plurality ofuser terminals obtains each of the plurality of data fragments of saiddata.
 14. A download system as in claim 13, wherein said delivery servercommunicates with said plurality of user terminals via a wide areanetwork.